FIELD OF THE INVENTION
The invention relates to a method of exchanging a nozzle passing through a cover of a nuclear reactor pressure vessel for guiding a control rod, which nozzle is fixed relative to the cover in a through-bore by a welded joint.
Such a nozzle penetration assembly is known from the journal entitled "Atomwirtschaft"[Atomic Industry], May 1975, pages 254/255.
In that article, a weld seam represents a load-bearing connecting link between the nozzle and the cover. Apart from the internal pressure, the welded joint must also be able to absorb lateral forces, on which, for example, an earthquake construction is based. An asymmetrical path of the weld seam, which is dictated by a penetration curve between the nozzle and the inside of the cover, results in disadvantageous stress loads on the nozzle, in particular underneath the welded joint. Since the inside of the nozzle has stresses caused by the weld seam, a medium contacting the inside, such as, for example, the coolant of a pressurized water reactor, promotes the occurrence of stress crack corrosion.
Therefore, after lengthy operation of a nuclear reactor plant, the occurrence of small incipient cracks at the nozzle, in particular in the region of the weld seam, cannot be ruled out.